delegating authentication to another service.
one of the more commonly known options would be sign in with google, but this is also quite useful for providers hosting multiple services. a provider could host a service that handles authentication and then you only have to login once and will automatically get logged in for their lemmy, xmpp, wiki and other services they might be providing.
I think most (especially mobile) clients simply don't have this option and will always copy/share the "fedi link" - the url where the content is canonically hosted. all other URLs are simply cached representations of the original content.